D3.1 phase 4: Remove the twelve moved fields from CustomUser (contract)

Part of the D3.1 umbrella (#334): phase 4 of 6. Contract step: removes the twelve moved fields and the moved unique constraint from CustomUser now that phases 2 and 3 have switched every reader.

Normative sources

Goal

CustomUser no longer declares role, certificate_name, country, region, registration_role, github_url, linkedin_url, personal_website_url, about_me, dark_mode, normalized_email, identity_state, or the accounts_active_normalized_email_unique constraint. courses.LearnerProfile and accounts_ext.IdentityState are the sole source of truth. No behavior change to any reader — they were already reading the new models as of phase 2/3.

Scope

• Migration removing the twelve fields and the constraint from CustomUser.
• Full-repository grep confirming no remaining attribute access to the removed fields off CustomUser/the auth model, outside migrations and the accounts.services.cmp_learner_import source-table string literals (accounts_customuser) and CmpLearnerImportProgress.table values, which name the CMP export's source tables, not the live model, and are intentionally unaffected.
• Re-run of the gallery-module regression test added in phase 2 to confirm it is still green after the fields are gone (it should never have depended on them).

Non-goals

• No rename (phase 5).
• No AISL field additions (phase 6).
• No change to username, newsletter_subscribed, home_dismissals, or newsletter_preference_changed_at (the last added to CustomUser after #334's original grooming, alongside newsletter_subscribed in the BE-15 fix) — all four stay untouched; their eventual disposition is D3.2/C3.7 adoption-time reconciliation, not this issue.

Dependencies

• Depends on: #391, #392 (both reader-migration phases must be merged and verified first).
• Blocks: #394 (rename cannot safely land while stale fields still exist to shadow the new profiles in a partial rebase).

Acceptance criteria

Django and integration

• make django-check / make migrations-check pass.
• make test (full suite) passes.
• CustomUser no longer declares the twelve fields or accounts_active_normalized_email_unique; the constraint is enforced on accounts_ext.IdentityState only.
• A documented grep over tracked .py files (excluding migrations and the named CMP source-table literals) shows zero attribute access to the removed fields off the user model.
• manage.py migrate applies cleanly end to end on a fresh database; the removal migration reverses, or its irreversibility is documented in the migration file.

Repository and operations

• P14 row-count rehearsal on a development database copy: counts before and after recorded, every difference explained by the phase 1 per-user profile tables plus this phase's column drop. If no development copy is reachable locally, record Not run here, needs: P14 rehearsal on a development database copy and keep the issue open with the human label at acceptance.
• Versioned verification plan per _docs/ci/change-selective-ci.md recorded in the engineer handoff.

First verify that phases 2 and 3 (#391 and #392) are merged, then read the CustomUser definition and the migration history alongside docs/plan/phase-3.md and _docs/specs/09-migration-rollout-roadmap.md. Run make django-check, make migrations-check, and the gallery regression test before making the removal migration; done means the full suite, fresh migrate, documented grep, reversal decision, and P14 handoff requirements are satisfied.